diff --git a/libpng-manual.txt b/libpng-manual.txt
index ea39bd4d0..0f549a5f3 100644
--- a/libpng-manual.txt
+++ b/libpng-manual.txt
@@ -611,7 +611,7 @@ callback function:
 
       /* also ignore unused known chunks: */
       png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
-         (int)sizeof(unused_chunks)/5);
+         (int)png_sizeof(unused_chunks)/5);
     #endif
 
 User limits
diff --git a/libpng.3 b/libpng.3
index 1883aeeb3..197c3e0dc 100644
--- a/libpng.3
+++ b/libpng.3
@@ -1103,7 +1103,7 @@ callback function:
 
       /* also ignore unused known chunks: */
       png_set_keep_unknown_chunks(read_ptr, 1, unused_chunks,
-         (int)sizeof(unused_chunks)/5);
+         (int)png_sizeof(unused_chunks)/5);
     #endif
 
 .SS User limits